Pulsed-neutron formation evaluation tools interrogate the formation surrounding the borehole with high-energy neutrons produced by a neutron generator forming part of the tool. Because of interaction by the neutrons with elements of the tool, with the borehole, and with the formation, gamma radiation is created, which is then measured by gamma radiation sensors that also form part of the tool. Measurement data captured by the gamma radiation sensors can be processed to derive information about the properties of the borehole and the subsurface formations.
Typically, the energetic neutrons are produced by a fusion reaction caused by generating an ion beam and accelerating it into collision with a target dosed with target molecules or atoms. The fusion reaction is often a deuterium-tritium reaction. Most often, the mechanism used to generate the ion beam functions fundamentally on the principles disclosed by Penning in U.S. Pat. No. 2,211,668.
In such ion sources, a high voltage pulse is applied between an anode and a cathode to create an arc discharge that initiates ionization of an ionizable gas exposed to arc discharge.